Fly-Free Zone

The Hessian fly is about as welcome to the wheat field as an ex-relative is to a family reunion. Like it or not, the old troublemaker is making a comeback on the list of pests to watch for this season.

Most people consider Hessian fly an insect dealt with in the fall. In general, fall seeding after the "fly-free" date provides adequate control. However, this practice appears to be breaking down, resulting in the spring infestations.

Kansas State University entomologist Phillip Sloderbeck says some growers are simply ignoring the fly-free date when planting. Occasionally frost and onset of cold weather are late enough to allow infestation of small grains in the fall, which overwinter and cause spring issues.
"Lack of crop rotation is probably a bigger issue for us," Sloderbeck says. "The current emphasis on soil and moisture savings plus the high price of fuel is encouraging reduced- and no-till farming. In Kansas, an increase in continuous wheat is leading to more Hessian fly problems."

University of Kentucky entomologist Doug Johnson also put Hessian fly on his watch list. "Warm fall temperatures and warm winters have caused a slight increase of this pest over the past few years," he says.

"The fly-free day helps, but in the end it is an axiom built on temperature and the premise of shutting the insect down with cold weather," he explains. "The trend toward warm falls has some people wondering if we need to re-evaluate the fly-free date."

Weather or calendar? The date has always been a little sketchy in the South, where temperatures are always more moderate. Ed Twidwell, Louisiana State University entomologist, says the state saw more Hessian fly problems when wheat acreage increased in response to high commodity prices.

In Louisiana, planted wheat acreage has dropped to around 200,000 acres for the 2009 season. "Wheat is often used for wildlife plots," Twidwell says. The plots are rarely managed with the same care as fields and can contribute to a "green bridge" for the pest.

Johnson worries most when volunteer wheat enters the picture. In this case, seed lies dormant during a dry summer and is activated by late-season rain. It becomes heavily infested when it emerges before the fly-free date and is exposed to long periods of egg laying by Hessian fly adults. The eggs deposited in volunteer wheat hatch into maggots that feed on the wheat and, when grown, change into the overwintering stage called flaxseed.

When the adults emerge from the flaxseed in the spring, they seek a host upon which to lay their eggs. The volunteer wheat is likely to be unthrifty, so the insects seek a more robust host, with production wheat the prime target. The exception is in the South, where the insect can sometimes go through three or more generations, rather than the two (fall and spring) that are normal for the Midwest.

Timely control. Johnson says it's not easy to decide if volunteer wheat will increase Hessian fly problems in the spring. "The insect is not a great flyer. The best guess is production wheat within 400 yards of heavily infested volunteer wheat is at increased risk."

Foliar-applied insecticides can control spring infestations, but they must kill the adults or the very young maggots before they move under leaf sheaths. Research from North Carolina and Georgia indicates that a well-timed application of a long residual synthetic pyrethroid will control both adults and young larvae. To control the spring adults, check the flaxseed or pupal stage in infested shoots of volunteer wheat by squeezing. If they appear orange, it is time to act. "You need to apply an insecticide just as adults are about to emerge," Johnson says.

Looking for the tiny transparent Hessian fly eggs is another method. They will be laid end-to-end in a row between leaf veins on the upper surface of wheat leaves. "Treatment may be justified if there are four or more eggs per leaf," Johnson says.

There are a few wheat varieties resistant to Hessian fly, but Sloderbeck says many are not highly resistant to wheat diseases. "The best control is prevention," he says. "Never allow a green bridge—aphids and wheat curl mites also use volunteer wheat as a staging ground to attack new wheat stands. Destroy volunteer wheat and avoid using wheat as a cover crop."